In the field of aerospace, the use of strong and light weight composite materials is increasing. A notorious drawback of these materials is that the process of their failure is rather complex, and damage may develop virtually invisibly from within composite structures, such as aircraft wings and wind turbine blades. To prevent accidents, regular manual inspection of these structures may therefore be required.
It has been suggested to automate the surveillance of mechanically loaded structures by means of an active health monitoring system including a fiber Bragg grating (FBG) sensor that is interrogated by an arrayed wave guide (AWG) based interrogator, which system may infer the existence of various kinds of damage to the monitored structure from changes in the Bragg wavelength. In such health monitoring systems the initial Bragg wavelength of an FBG sensor may typically be situated between the respective center wavelengths of two associated output ports or output channels of the AWG, such that a response provided by the FBG sensor upon interrogation is divided between those ports. When, during use, the Bragg wavelength of the FBG changes due to the fact that it is strained, the power ratio between the two output ports also changes.
One drawback of the above-described configuration of an FBG/AWG health monitoring system is that the sensitivity of the system is greatest when the initial Bragg wavelength is located precisely between the center wavelengths of the two associated output ports of the AWG. It is rather difficult, however, to effect this condition on installation. Accordingly, a freshly installed health monitoring system may generally have a less than optimal sensitivity. Another drawback is that an FBG sensor may preferably have a relatively large dynamic range over which its Bragg wavelength may shift during use. Consequently, a Bragg wavelength may easily shift outside the spectral range of at least one of the output ports, which may render an accurate wavelength determination impossible.
To mitigate the aforementioned drawback, the applicant invented a fiber Bragg grating interrogator assembly with spectrally overlapping output ports, and a corresponding method of interrogating a fiber Bragg grating, described in the Dutch patent NL 2010940.
It has been found that such a fiber Bragg grating interrogator assembly can be made substantially polarization independent, however, for example due to production variations, particularly of the optical waveguides, there remains a relatively small polarization dependent response.